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False
A postulate.
Assumption
The initial assumption is "that it is NOT a right angle"
Given a proposition X, a regular proof known facts and logical arguments to show that X must be true. For an indirect proof, you assume that the negation of X is true. You then use known facts and logical arguments to show that this leads to a contradiction. The conclusion then is that the assumption about ~X being true is false and that is equivalent to showing that X is true.